1. Technical Field
The present invention relates to an electrostatic capacitance fluid level sensor detecting an amount of fluid, and more particularly to an electrostatic capacitance fluid level sensor having integrally formed hermetic terminals via which the sensor is mounted on an electrically driven compressor and the like.
2. Related Art
Methods of detecting an amount of fluid have been conventionally known. Electrostatic capacitance sensors have been proposed as means for detecting an amount of electrically nonconductive fluid such as oil. This sensor detects the presence of fluid and an amount of the fluid, using changes in electrostatic capacitance between electrodes by causing electrically insulating fluid to penetrate into a space between the electrodes. For example, the electrodes are disposed in a cylindrical casing which is electrically insulating or one of the electrodes is formed into a cylindrical shape. When fluid flows into and out of the cylindrical member, electrostatic capacitance between the electrodes changes. An amount of fluid in the container is measured from the changing electrostatic capacitance.
The electrostatic capacitance fluid level sensors include a type which can detect continuous changes in an amount of fluid on a real-time basis as shown in the foregoing prior art documents. However, the determination to detect a subtle change in the electrostatic capacitance becomes more difficult as the measurement is precise. Furthermore, since the determination changes depending upon a type of liquid and/or a type or size of the container to which the sensor is mounted, actual gauging is required.
For example, FIGS. 6 to 8 show a proposed example of fluid level sensor having a simplified construction by limiting the use of the sensor only to detection of presence of fluid but not a continuous detection of fluid amount. The fluid level sensor 101 includes a base 102 to which two conductive pins 104 are fixed. The pins 104 have distal ends to which metal electrode plates 105 are fixed by welding and configured to face each other substantially in parallel with each other.
While the above-described fluid level sensor has a simple structure, a slight tilt of each electrode plate caused by the welding to the conductive pin changes an interelectrode distance. Accordingly, adjustment of interelectrode distance is necessary in order that the tilt caused during the welding may be removed for the purpose of uniforming the sensitivity. In particular, a slight tilt of the welded part has a large influence on the interelectrode distance as the electrodes are rendered long so that areas of the electrodes are increased for the purpose of obtaining electrostatic capacitance. Furthermore, since the cantilevered electrodes are bared, there is a possibility that the electrodes may be deformed when subjected to a force during assembly. Furthermore, when the strength of the electrodes is increased by thickening the electrodes for prevention of deformation, stress concentrates on welded parts which are relatively weaker. This has a possibility of changing the positional relationship of the electrodes.
Even when electrodes are adjusted to a proper positional relationship in manufacture, an increase in the internal pressure expands the base in the case where the sensor is mounted to a motor compressor. This slightly changes an angle between electrically conductive pins, resulting in a large change in the distance between electrode plates mounted on the distal ends of conductive pins. Accordingly, the electrostatic capacitance between the electrodes changes with the result of a problem that the performance of the fluid level sensor is reduced.
Furthermore, since two pieces of electrode are welded on the same straight line connecting between the conductive pins, one welding electrode needs to be inserted between the conductive pins. Furthermore, the firstly fixed electrode gets in the way of the welding of the second electrode. Additionally, the interelectrode spacing and the electrodes become slenderer and thinner as the sensor is small-sized, with the result of a problem that a sufficient pressure applied between the electrodes during welding cannot be obtained.
Accordingly, an electrostatic capacitance fluid level sensor is desired which is simple in the construction and easy to manufacture and handle.